Wednesday, July 10, 2013

Air conditioning not working as well as expected

Every
once in a while I come across an air-conditioning system that does not
seem to be working properly. No matter how low the thermostat is set,
there doesn’t seem to be any cold air coming out of the registers.

Lack
of air from the registers can mean a number of things, ranging from a
failed fan in the furnace, to leaking ducts or duct baffles closed. But
one of the biggest causes is ice build-up in the system.

The main air conditioning system is a combination of components as can be seen from the diagram here:

Outside
the home, there is normally the thing most people call the air
conditioner unit. This is in fact only a piece of the puzzle. This
unit is a combination of a compressor, a large coil, not unlike a
radiator in a car, and a big fan. The proper name for this unit is the
condenser unit.

Coming
from the condenser unit are two, usually copper, pipes. One will be
insulated, and the other, generally not. These pipes go through a hole
in the wall, and into the tin box above the furnace. This is known as
the plenum. Inside this box are the final parts of the Air conditioning
system, the evaporator coil and the evaporator drain lines.

So how does this all work?

Bear with me, as the next bit is a bit technical, so I’ll try to make it as simple as possible.

When
you compress a liquid, it heats up, and it wants to boil, and turn into
a gas. If you cool the gas down it condenses back into a liquid, but
still under pressure. If you release the pressure very quickly, the
boiling point drops, and the liquid evaporates, and the temperature
drops rapidly also.

This
can be demonstrated by firing off a carbon dioxide fire extinguisher.
Inside the extinguisher is carbon dioxide liquid at high pressure. The
normal boiling point of Carbon Dioxide is minus 78.5c, so at room
temperature it’s a gas. By compressing it, and cooling it, it turns
into a liquid, which is then kept under pressure in the fire
extinguisher.

When the pressure is released, the carbon dioxide drops rapidly in temperature, and turns into dry ice.

To
test this theory for yourself, place some warm water on the back of
your hand and blow on it. Instead of heating your hand, you will feel
your hand cool down as the water evaporates.

It
is a combination of these things that helps air conditioners work.
Pressurising a gas, and then cooling it to form a liquid. Transporting
that liquid to somewhere else, where the pressure is released and the
liquid evaporates and cools down, and then returning it to where it can
be re-pressurized.

So
going back to the diagram, the A/C refrigerant, is pressurised in the
compressor, where it heats up as a gas. It is then cooled by blowing
cold air over the condensing coils from the big fan until it condenses
into a high-pressure liquid. Liquid under pressure will try to find
it’s way to a lower pressure, so the liquid flows along the pipe, until
it gets to the really low pressure in the evaporator coil. Here the
pressure drops very quickly, and the liquid evaporates into a gas again,
cooling down as it does. This is transferred to the air by the
evaporator coils and is blown around the house by the furnace fan.

Warm,
moist air traveling over a cold surface will cause condensation, which
inside the plenum of the HVAC system turns into water and is drained
through the evaporator drain lines. The warmer gas then returns to the
compressor to start its cycle all over again.

So what can go wrong?

The
temperature drop at the evaporator coil dos controlled by the pressure
of gas in the system. Too much, and the gas will not cool enough, so
the A/C will be ineffective. This is unlikely to ever be the case, as
there is a pressure relief valve at the compressor on the condenser unit
to release gas that is over pressure.

When
there is not enough pressure however, this is when things get
interesting. The gas is compressed much further, because there is less
initial pressure in the system. This then starts to decompress around
the system, and when the gas reaches the expansion device and evaporator
coils it evaporates so quickly, because there is less pressure in the
system, that any moisture on the outside of the coils freezes.

This is much like the frost build-up you see on the coils inside a refrigerator’s freezer box.

The
fan inside the furnace, keeps supplying warm moist air to flow over the
evaporator coils, which creates condensation as it hits the colds
coils, and then because of the extreme low temperatures freezes.

This continues until the coils are completely frost laden, and the air flow then stops.

The
picture on the right shows an evaporator coil that has completely
frozen. This stops all airflow through the system, so even though
there’s plenty of cold, there’s no cold air.

The problem is this
is hidden from view, and so you are not likely to be able to tell if
this has happened inside the plenum of the furnace, but there are other
tell-tale signs.

The first is the lack of airflow at the
registers when the air conditioning is on. Even with the fan at full
flow you will feel little airflow from the registers, and the upstairs
registers, normally the furthest away may have no air coming from them
at all.

The second tell-tale is and ice formation around the
high-pressure valve of the condenser unit outside. This is a good
indication of low pressure in the gas and excessive de-pressurisation in
the system causing ultra low temperatures in the pipes and coils.

You can see an example of this on the photo to the right.

How do you fix this?

It’s
going to require a service call to have an HVAC engineer come out and
re-pressurize the system. At the same time they can check for leaks
that may have created the low-pressure in the first place. BUT WAIT!
Before calling out the HVAC engineer, it is important you shut off the
A/C for at least 4-5 hours to let the ice melt. The engineer cannot work
on the HVAC system f its frosted up, so this will just cost you a call
out fee, the problem won’t get fixed and you’ll have to arrange a second
call-out.

During
the period that the a/c is off, keep an eye on it. There’s a lot of
ice that’s going to melt, and you want to ensure it comes out of the
drain lines, and not just pour down over the electronics in the furnace
below.

Why does the refrigerant pressure drop?

There
are three reasons that refrigerant pressure is lost. The first is from
a leak. These systems are designed to be very secure with respect to
leaks, because the refrigerant is not environmentally friendly. Older
systems can leak, but the cause of a leak is usually a valve or fitting
not being tightened properly by the installer.

The
second reason for a pressure drop is refrigerant theft. This is less
common in Canada than in the U.S. but it does go on. Refrigerant gas is
not cheap, and the resale value is high.

The
third and most worrying reason for pressure drop is that it may be
being consumed as a drug. Unprotected refrigerants are easily
accessible with a simple pair of pliers or fingers, and the thieves can
sniff the refrigerant directly from the valve or store it in a plastic
bag for later use. Unfortunately, the “high” or “rush” an individual
gets from “huffing” is from the oxygen being displaced in their body.
This has resulted in documented cases of brain injury and even death to
individuals.

If
the HVAC engineer cannot find a leak when they are re-pressurizing,
then you should ask if they can fit a tamper resistant cap to the
filling valve. They are readily available and not expensive when
compared to the call-out fee and replacement refrigerant needed to get
you’re a/c working again.

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